1. Field of the Invention
This invention pertains to linings for use in ordnance items, and more particularly pertains to such linings which improve the ability of an ordnance item to survice prolonged exposure to a high temperature environment.
2. Description of the Prior Art
Structures which contain heat sensitive energetic materials, such as rocket motor combustion chamber casings which enclose propellant grains, or warhead casings which enclose explosive material, will explode when exposed to a fire or other high temperature environment because of ignition of the energetic material and resultant catastrophic thermal stress and internal pressure buildup. Military test procedures and requirements for the heat resisting capability of ordnance items are described in Mil-Std-1648, a military specification of the United States Government which is unclassified and available to the public.
One place where the problem of ordnance exposure to fire is especially serious is on the flight deck of an aircraft carrier where a large quantity of explosive ordnance may be temporarily stored or attached to aircraft during flight operations. In such situations it is possible for an aircraft fuel tank to rupture and cause a fuel fire on the carrier flight deck. This fire will engulf ordnance loaded on aircraft or stored nearby, and will eventually cause that ordnance to explode with devastating effect upon surrounding equipment and personnel.
Past attempts to solve this problem have included placing a thermal barrier on the exterior of the rocket motor casing or warhead in an attempt to thermally insulate the rocket motor propellant or explosive material from the fire. This approach attempts to increase the length of time the ordnance item may be exposed to a fire without exploding, by keeping the internal temperature low. If the fire is not extinguished within a short period of time, the ordnance item ignites and explodes. Such thermal insulating coatings are not efficient because they add nonfunctional weight to the rocket or warhead as well as increase the cost and field handling problems associated with that ordnance item. Also, aerodynamic drag is increased.
Pressurization liners have also been used with varing degrees of success. Such liners are designed to degrade into a gas at low temperature to supply controlled internal pressure for causing rupture of preweakened venting structure in the casing wall. The main problem encountered with pressurization liners is that if the liner degrades and pressurizes too suddenly, the propellant grain may crack and explode anyway. Most liners used, whether for pressurization or other purposes, will degrade at a relatively low temperature.
Intumescent coatings have also been used with varying degrees of success. The main drawback to using an intumescent coating is that the coating is applied to the exterior of the item and must be protected from scratches and abrasion. When exposed to a fire, the intumescent coating forms a very weak structural insulating layer which may easily be swept away if, for example, a stream of water is directed upon it. The ordnance item is then directly exposed to the fire.